Electrochemical Preparation of Fine Powders of Nickel-Boron Alloys in Molten Chlorides for Magnetic Hydrogenation Catalysts

Abstract

Fine powders of magnetic nickel-boron alloys were prepared for the first time by direct electrochemical reduction of the 2Ni 3 (BO 3 ) 2 /B 2 O 3 , Ni 3 (BO 3 ) 2 /NiO and Ni 3 (BO 3 ) 2 /3NiO composite precursors in molten CaCl 2 -NaCl at 700• C. Precursors with different nickel to boron atomic ratios (R Ni/B = 1, 2 or 3) were synthesized by the sol-gel process followed by thermal annealing at 750• C. The reduction pathway, phase transformation and morphology of the samples electrolyzed at different voltages and times were analyzed by XRD, SEM, and EDX. When the applied voltage reached to 3.0 V, pure Ni 3 B can be obtained from the oxide precursor with R Ni/B = 3. However, the oxide precursors with R Ni/B = 1 and 2 both resulted in mainly Ni 2 B. Metal borides with a boron covalent framework have interesting physical properties, such as high hardness, superconductivity, thermal stability, oxidation resistance, high melting points and outstanding permanent magnetism. They are widely used in various high-tech industries.1 Metal borides also possess high catalytic activity to the hydrogenation reactions and hydrogen production.2-4 Metal borides are prepared in three forms, namely, the bulk materials, the amorphous or crystalline powders. Each of them has its applied advantages. For example, nano-amorphous nickel borides have shown much higher activity and selectivity in the catalytic hydrogenation of organic substances such as benzene, p-chloronitrobenzene, and furfural. [5][6][7] The crystalline nickel borides can be used as diffusion barrier for Cu integrated circuit, 8 and also used as surface coating for the strengthening or corrosion prevention of the substrate. 9-10Although nickel borides can be easily prepared by the high temperature reaction between Ni and B, 11 it is quite difficult to convert the obtained bulk borides into fine powders. Nanometer Ni-B can be prepared by reaction between the borohydride and the nickel ions in a solution. For example, nano-amorphous Ni-B was synthesized in aqueous solutions, 12-13 and nano-crystalline Ni 3 B was prepared at 300• C in tetraethylene glycol. 14 Alternatively, the solvothermal method in benzene resulted in nano-crystalline Ni-B as well.15 However, these methods need expensive and toxic borohydrides serving both as the reducing agent and boron source.Since late 1990s, it has been reported that metal, alloy and intermetallic powders can be directly prepared by electrolysis of solid precursors of metal compounds (e.g. oxides or sulfides) and mixed metal compounds respectively in high temperature molten salts. [16][17][18][19][20][21][22][23][24] This method is also capable of processing and production of nanomaterials for various applications such as enhancing catalytic and photoresponsive performances 17,25-28. Metal borides could be also prepared by this process. However, the melting point of B 2 O 3 (450 • C) is much lower than the working temperatures (600• C ∼900 • C) of the commonly used molten chloride salts such as CaCl 2 and LiCl. Therefore, B 2 O 3 c...

Direct Electrochemical Preparation of Ni-Cr Alloy from Solid Oxides in Molten CaCl2

“…Since the FFC-Cambridge process uses the electric current to remove the oxygen from the cathode, the current efficiency can be estimated from the following relation: assuming the weight losses is due to the removal of oxygen from the pellets, the current efficiency can be calculated according to the measured mass loss of the pellets and the mass loss, due to removal of oxygen, calculated from the total electrolysis charge. 22 Hence, the obtained current efficiency is 50% and the energy consumption for the production of Cr 1.12 Ni 2.88 is about 6.66 kW h/kg. The current efficiency could be even higher if the As the phase changes with the electrolysis progresses, the morphologies of the particles in the pellet change dramatically and nodular particles of porous structure observed to be sintered lightly are synthesized after 4 h electro-reduction.…”

“…[4][5][6] This process has critical shortcomings such as its being hard to produce a homogeneous alloy composition as a result of the large melting point (Ni, 1453 • C; Cr, 1875 • C), density (Ni, 8.9 g/cm 3 ; Cr, 7.19 g/cm 3 ) difference and a complicated production process that requires significant amounts of energy and cost. [4][5][6] The recent electrochemical reduction, namely the FFC-Cambridge process, of pure or mixed oxide powder to metal or alloy in molten salt offers an alternative technology for the metallurgical industry, and so far has been demonstrated for successful production of transition metal and alloy including Ti, [7][8][9] Cr, [10][11][12][13][14][15] Ta, 16 Cr 7 C 3 , 17 La 2 (Ni 1-x Co x ) 7 (x = 0.05, 0.1, 0.2), 18 Ni-Nd, 19 Co-Cr, 20 Fe-Ti, 21 and nickel-boron, 22 et al O 2− is released from the solid oxide cathode and diffuses toward the graphite anode through the molten salt, which discharges at the graphite anode and liberates as CO or CO 2 under the influence of the applied voltage and the high temperature of the molten salt. 23 This method is a much easier, less expensive and more environmentally friendly process.…”